logic.erl

-module(logic).
% In the previous video step on pattern matching we saw two ways of defining 
% “exclusive or”. Give at least three others. You might find it useful to 
% know that:
% =/= and == are the operations for inequality and equality in Erlang;
% not is the Erlang negation function; and,
% and and or are the Erlang conjunction and disjunction (infix) operators.
-export([exclusive_or_1/2,exclusive_or_2/2,exclusive_or_3/2,exclusive_or_4/2,
exclusive_or_5/2, max_three/3, how_many_equal/3, test_exclusive_or_1/0]).


exclusive_or_1(true,false) ->
  true;
exclusive_or_1(false,true) ->
  true;
exclusive_or_1(_,_) ->
  false.

exclusive_or_2(X,X) ->
  false;
exclusive_or_2(_,_) ->
  true.


exclusive_or_3(Left, Right) ->
  Left =/= Right and 
  ((Left==true) or (Left==false))and 
  ((Right==true) or (Right==false)).
  
exclusive_or_4(Left, Right) ->
  (Left and not Right) or (not Left and Right).

exclusive_or_5(Left, Right) ->
  not ((Left and Right) or (not Left and not Right)).

max_three(First, Second, Third) ->
  max(First, max(Second, Third)).

how_many_equal(X,X,X) -> 3;
how_many_equal(X,X,_) -> 2;
how_many_equal(X,_,X) -> 2;
how_many_equal(_,X,X) -> 2;
how_many_equal(_,_,_) -> 0.

test_exclusive_or_1() ->
  true = exclusive_or_1(true, false),
  true = exclusive_or_1(false, true),
  false = exclusive_or_1(true, true),
  false = exclusive_or_1(false, false),
  all_good.

Nicely done!